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National Water Summary 1988-89-Floods and Droughts: COLORADO 213 ~ several major river basins. The assumption was made that this period of large runoff ended the 1949-57 drought. The 1930--42 drought was regional in scope. The recurrence interval was 25 years or more statewide and 10-25 years in some small areas. Annual-departure graphs for most sites show predomi- nantly less than average streamnow (fig. 4, sites 1-3, 5,6). The length of record of the Colorado River near Cameo (fig. 4, site 4) is insufficient to define the drought. Other gaging-station records, however, indicate that the drought was severe in the Colorado River basin. The Great Plains in Colorado were more severely affected by the 1930--42 drought than elsewhere in the State, because of the "dust bowl," which was caused by the combination of drought, increased tillage, and strong winds. Agricultural losses were substantial throughout the Great Plains in Colorado. The 1949-57 drought affected the entire State. The drought had recurrence intervals of between ] 0 and 25 years in the lower Arkansas River basin; but in other areas, ~treamflow deficits indi- cated a drought with a recurrence interval of greater than 25 years. Thomas and others (1963, p. F-l) reported that. in the Colorado River basin in Colorado, the drought initially was not as severe as in the basin downstream from Colorado. However, the severity of the drought increased in the upper basin in 1952, and Colorado was then considered to be in the area of drought. During 1958-70, a severe drought with a recurrence interval greater than 25 years affected most of the State. Two small, isolated drainage basins were not affected or were affected by droughts having recurrence intervals of less than 10 years. Several other basins or parts of basins were affected by droughts having recurrence intervals that ranged from 10 to 25 years. The duration of the 1958-70 drought ranged from about 6 to 12 years at the selected gaging stations (fig, 4, sites 1-6), The severity and duration of the 1976-82 drought were more variable than those of the previous major droughts. In the descrip- tion of the hydrologic and human aspects of this regional drought, Matthai (1979, p. 1) considered the drought to last 2 years, 1976- 77: however, the analysis of discharge at 50 gaging stations indicated that the minimum duration was slightly longer than 2 years, and the maximum was as long as 13 years. WATER MANAGEMENT -' The population of Colorado is projected to increase from about 3.4 million in 1988 to more than 4 million in the year 2000 (William P. Stanton, Colorado Water Conservation Board, written commun., 1988). Associated with this increase will be the potential for flood- plain development and for municipal and industrial development that will require an adequate long-tenn water supply. Without proper consideration of flood and drought hazards. the variability of nature occasionally will cause tragic social and economic losses. Flood-Plain Managemenl.- The Colorado Water Conserva~ tion Board (1985, appendix C) is responsible for flood-plain man- agement at the State level of government, as set forth by the Colorado Revised Statutes. The Board also is the State coordinating agency for the National Flood Insurance Program and, as such, maintains copies of the maps used to administer the program. Flood-plain information for Colorado is published by nu- merous Federal, State, and local government agencies and by some private corporations, developers, and individuals. About 150,000 people, or about 5 percent of the State's population, are estimated to live in an area of a nood plain that would be inundated by a discharge having a recurrence interval of 100 years. The total value of property exposed to flood hazard is estimated to be $6 billion (Colorado Water Conservation Board. 1985, p. i). To cope with flood problems in Colorado, flood-plain man- agers use various strategies. Development is directed away from the hazard through enforcement of comprehensive flood-plain manage- ment ordinances and building codes at the local level. Any new construction allowed on the flood plain must be at least 1.0 foot above the predicted altitude of a flood having a recurrence interval of 100 years (Colorado Water Conservation Board, 1985, appendix C). Methods used to protect existing development from flood- waters include channelization, construction oflevees, and construc- tion of reservoirs having the capacity to retain floodwaters. Major levees have been built in Alamosa, Grand Junction, Greeley, Las Animas, Pueblo, and Wiggins. Major flood-control reservoirs include Cherry Creek Lake on Cherry Creek, Chatfield Lake on the South Platte River, Bear Creek Lake on Bear Creek, Trinidad Lake on the Purgatoire River, and John Martin Reservoir on the Arkansas River. Other methods used in Colorado to control flood losses in- clude education of the public, purchase of flood insurance through the National Flood Insurance Program, and response to flood- warning systems. However, only an estimated 9 percent of all structures within Colorado's flood plains are insured, and the insur- ance coverage is only about 7 percent of the value of the exposed property (Colorado Water Conservation Board, 1985, p. vii). Flood-Warning Sysrems.-The Office of the State Engineer, Colorado Division of Water Resources, in cooperation with the Central Forecast Office of the National Weather Service in Denver, operates a statewide flood-warning system. The system consists of 78 gaging stations that are operated by both State and Federal agencies. These gaging stations all have satellite-linked monitoring equipment installed with a receiving site operated by the Office of the State Engineer. The Central Forecast Office, which operates on a 24-hour basis, is automatically alerted by the system if streamnow rises rapidly or if a specified threshold stage is reached. Ifconditions warrant, either a flood watch or a flood warning is issued (Colorado Division of Water Resources, ] 988). Several sophisticated "flash flood" warning systems also have been installed and are operated by local governments on specific streams. These sysrems are located along the eastern foothills of the Rocky Mountains in areas of dense population. These systems typi- cally consist of a network of automatic telemetry rain gages and streamf10w gages supplemented by volunteer weather observers and radar observations. Water-Use Management During Droughts.-The Colorado Drought Response Plan was created during a drought that was de- veloping in early 1981. The plan identifies two distinct and sepa- rate management functions during drought: assessment and response (Colorado Division of Disaster Emergency Services, 1981). Assessment is the responsibility of the Water Availability Task Force, which monthly reviews various indicators such as the Palmer Drought Index produced by the National Weather Service and the Surface Water Supply Index developed by the Office of the State Engineer and the U.S. Soil Conservation Service (D .S. Soil Conser~ vation Service, 1988, p. 18). If the index is less than an established numerical criterion indicating a potential shortage, one or more impact task forces are activated to study the situation in greater detail and to assess the potential effects. Personnel of various State agencies, which are identified as having responsibility for action, respond to the impact of drought. If drought conditions worsen beyond the jurisdiction of personnel of the lead State agency, an Interagency Coordinating Group is acti- vated to review unmet needs and to bring major problems to the attention of the Governor and the legislature. SELECTED REFERENCES Colorado Division of Disaster Emergency Services. 1981. Colorado drought response plan: Denver. III p. Colorado Division of Water Resources, 1988. The Colorado satellite-linked water-resources monitoring system, annual status report, fiscal years 1986--87, 2d ed.: Denver, 158 p.